This invention relates to processes in which a plate or press must be held in a fixed position during a wide range of thermal cycling such as in the production of expanded thermoformable materials and products in general and more particularly to an improved method and apparatus for maintaining a press or plate so fixed. With respect to producing thermoformable products, it permits producing larger panels of such materials than was heretofore possible.
In general terms, an important type of process and apparatus to which the present invention relates is that disclosed in a series of U.S. Pat. Nos. 3,919,445; 3,919,380; 3,919,381 and 3,919,378, all issued Nov. 11, 1975 to Walter H. Smarook. Basically, as disclosed in U.S. Pat. No. 3,919,445 the process is one in which a blank of thermoplastic polymeric material is placed between two molds of a heated press. The temperature of the mold plates is about 5.degree. to 10.degree. C. above the temperature at which the thermoplastic material exhibits hot tack adhesion. The mold plates are separated apart and the adhesive forces of the polymeric material to the mold plates are greater than the cohesive flow properties of the polymeric material itself during the plate separation or expansion step. Thus, it is possible to mechanically move the mold plates a certain distance apart with the polymeric material bonded to the surfaces thereof without causing a rupture of the adhesion between the surfaces and the fused material. The plates are separated causing the panel to take on a new internal geometry and the plates then cooled to solidify the expanded panel. During the process it is mandatory that the plate surfaces be parallel to each other within a few thousands of an inch, first during the melting stage, or else the plastic adhesion will not become accomplished, and finally, during the cooling state, or else the final pattern will not meet commercial standards of thickness uniformity. Typically, temperature cycling can range between 60.degree. and 700.degree. F.; a more common range being from 100.degree. to 400.degree. F. Thus, a fundamental problem in this process is that the metal mold plates, when subjected to temperature changes, also undergo dimensional changes and, when fixed to a supporting frame which is, of course, necessary they tend to bend so as to lose their parallelism. In addition, buckling of the supporting frame can also occur. Most developmental work in these processes described in aforementioned patents was done on 6 inch squares. Maintaining parallelism with such a small size is not particularly difficult. However, attempts to make larger size panels, up until the time of the present invention, have not been particularly successful. Even panels in a size as small as 14 .times. 24 inches have proven problematic. There is, however, a need for larger panels, for example, panels of sizes up to and larger than 4 .times. 10 feet. For example, such panels can be used as floating roof covers in chemical and protroleum tanks to reduce pollution and evaporation loss. There are, of course, many other applications for such panels, some of which require high tolerances, particularly where a plurality of panels are bonded or fitted together, e.g. office partitions and desk tops.
Thus, it can be seen that there is a need for an improved method for carrying out an expansion process with thermoformable material so as to produce larger size products, such as panels, which are commercially acceptable.
In general terms, there is a need for a method for holding large presses or plates fixed during thermal cycling.